(1) Field of the Invention
The present invention relates to a metal hydride electrode mainly composed of a hydrogen-absorbing alloy, and particularly to such a metal hydride electrode to be provided for a sealed-type nickel-hydrogen alkaline storage cell.
(2) Description of the Related Art
Nickel-hydrogen alkaline storage cells have been attracting attention because of their advantages of being lighter in weight, larger in capacity, and higher in energy density than conventional nickel-cadmium storage cells and lead storage cells.
Such a sealed-type nickel-hydrogen alkaline storage cell is generally produced as follows.
First, the metal hydride electrode of the sealed-type nickel-hydrogen alkaline storage cell is produced as described in Japanese Laid-Open Patent Application No. 61-66366 as follows. Hydrogen-absorbing alloy powder is kneaded with a binder such as polytetrafluoroethylene (hereinafter PTFE) or polyethylene oxide into a paste, applied onto both surfaces of a support made of a punching metal or an expanded metal, and dried.
The metal hydride electrode thus produced is coiled together with a sintered nickel positive electrode via a separator, and put into an outer can. The outer can is sealed after an alkaline electrolyte is poured thereinto.
The sealed-type nickel-hydrogen alkaline storage cell thus produced is designed to have a negative electrode with larger capacity than a positive electrode in order to prevent the generation of hydrogen gas from the negative electrode in the final period of a charging operation. Consequently, the positive electrode falls into an overcharged state during a charging operation earlier than the negative electrode. In the overcharged state, oxygen gas is generated from the positive electrode through the following reaction. EQU 4OH.sup.- .fwdarw.2H.sub.2 O+O.sub.2 +4e.sup.- ( 1)
However, the oxygen gas generated from the positive electrode moves to the negative electrode through the separator, and reacts with hydrogen on the surface of the negative electrode composed of hydrogen-absorbing alloy which is being charged. As a result, water is generated. The reaction is shown in the following reaction formula (2). EQU 4MH+O.sub.2 .fwdarw.4M+2H.sub.2 O (2)
The oxygen gas generated from the positive electrode is thus consumed in the negative electrode, so that there is no raise in the cell internal pressure in the case of a sealed type cell.
However, if the reaction of the formula (2) is not smoothly performed due to the insufficient oxygen gas consumption of the negative electrode, the oxygen gas which is generated from the positive electrode and remains unconsumed is accumulated within the cell, thereby raising the cell internal pressure. The raise in the cell internal pressure causes the electrolyte to leak out of the cell together with the emission of the oxygen gas through a safety vent.
On the other hand, even if a storage cell is provided with a negative electrode having fairly high ability of absorbing oxygen gas, when the storage cell is charged at a high rate until being overcharged, a great amount of oxygen gas is generated from the positive electrode. As a result, the negative electrode cannot afford to absorb all the oxygen gas generated, and the oxygen gas begins to oxidize the hydrogen-absorbing alloy itself. The oxidization of the alloy leads to the deterioration of its hydrogen absorbing/desorbing ability. In short, the generation of too much oxygen gas leads to the decrease in the charge/discharge cycle life of the cell.
In order to avoid this problem, the following methods are used to lessen the oxygen gas to be generated from the positive electrode and to accelerate oxygen gas consumption at the negative electrode.
1) A method for controlling charged amount properly PA0 2) A method for reinforcing oxygen gas consumption ability of the negative electrode by adding a metal thereto PA0 1) The additive made of metallic oxide and/or metallic hydroxide which is applied on the surface of the electrode dissolves in the alkaline electrolyte more easily than when it is in the form of the metallic element. Therefore, the additive dissolves quickly in the form of metallic ions in the alkaline electrolyte. The metallic ions dissolved in the alkaline electrolyte reprecipitate on the surface of the electrode when the cell is charged. Since the conductive powder such as carbon powder is dispersed on the surface of the electrode, the conductive powder enlarges the surface of the electrode and also works as a crystal nucleation sites of the metallic crystal. Consequently, the metallic ions in the electrolyte can smoothly precipitate all over the surface of the electrode around the crystal nucleation sites. The metallic crystal particles uniformly precipitated on the surface of the electrode are small in size and have large specific surface area, so that the area concerned with the reaction with oxygen gas is large. Consequently, the oxygen gas generated from the positive electrode is effectively combined with the metal, thereby accelerating the oxygen gas consuming reaction of the negative electrode (hydrogen-absorbing alloy). PA0 2) The additive made of metal oxide or metal hydroxide having oxidation-reduction potential nobler than the operational potential of the hydrogen-absorbing alloy can enlarge -.DELTA.V in accordance with the principle of 3) below. A cell having large -.DELTA.V can perform a charging operation properly based on -.DELTA.V detection control system. As a result, overcharging can be prevented, and the decrease in a cycle life resulted from generation of too much oxygen gas can be prevented. PA0 3) The mechanism that the additive can increase -.DELTA.V is explained as follows.
When a nickel-hydrogen alkaline storage cell is charged, the cell voltage gradually increases until it reaches its peak which comes immediately before the full-charged state. If a charging operation is continued even after the peak, the cell is set in an overcharged state. During the time period from the full-charged state to the overcharged state, the cell voltage drops. The dropping of the voltage (-.DELTA.V) is detected, and upon detecting the -.DELTA.V, a charging operation is suspended to prevent the cell from falling into the overcharged state.
Thus, these kinds of cells are charged with the use of a charger capable of storing the peak voltage value at the final period of the charging operation and of suspending the charging operation at the point where a certain amount of voltage -.DELTA.V has dropped.
In the case of nickel-hydrogen alkaline storage cells, however, the amount of change in the cell voltage between the full-charged state and an overcharged state is too small to be detected accurately and speedy. This problem brings about a time lag between the full-charged state and the overcharged state, and overcharge is progressed during the time lag. As a result, a great amount of oxygen gas is generated from the positive electrode.
This method has been proposed in Japanese Laid-Open Patent Applications Nos. 2-239566, 5-41210, and 3-274664.
The application No. 2-239566 discloses adding a metallic oxide such as CuO to the inside of the negative electrode made of hydrogen-absorbing alloy. The application No. 5-41210 discloses adding at least one of a metallic oxide, a metallic hydroxide, and a metallic salt each of which can be either copper, bismuth, lead, silver, or thallium. According to the technique, a metal added to the negative electrode accelerates the oxygen gas consumption reaction of the negative electrode and increases the amount of voltage -.DELTA.V to some extent. Consequently, the charge/discharge cycle life of the cell can be improved.
However, these metals to be added to the negative electrode do not have the ability of absorbing/desorbing hydrogen, so that the energy density of the negative electrode is decreased in accordance with the amount of the added metal. As a result, it is necessary to prevent the decrease in the energy density by minimizing the amount of metal to be added to the negative electrode. The prevention requires to make the best use of the effects of accelerating the oxygen gas absorption of the metal. According to these applications, however, the effects of accelerating the oxygen gas absorption of the metal are not fully obtained.
On the other hand, the application No. 3-274664 discloses a technique of providing a layer made of metallic powder and carbon powder on the surface of the negative electrode made of hydrogen-absorbing alloy, in order to improve the ability of consuming oxygen gas.
However, this technique also fails to fully obtain an effect of accelerating oxygen gas absorption of a metal and another effect of increasing the voltage drop after the full-charged state.